1. Field of the Invention
The present invention relates to pulsed electromagnetic sensors, and more particularly to fluid and material level sensors using time-domain reflectometry (TDR). These sensors can be used for determining or controlling the fill-level of a tank, vat, irrigation ditch, silo, pile,. or conveyor. Also, the present invention can be used as a linear displacement transducer for use in machine control.
2. Description of Related Art
TDR techniques have been used in the past for measuring the fill-level in a tank. For example, U.S. Pat. No. 3,703,829, Liquid Quantity Gaging System, to Dougherty discloses a time-domain reflectometer (TDR) connected to a coaxial cable, or probe, immersed in a liquid, wherein the time delay of the reflected pulse is a measure of the liquid level in the coaxial probe. The key advantages to coaxial TDR probes are (1) strong reflection amplitudes, which are of particular advantage with low dielectric constant materials, and (2) stilling action, wherein sloshing is less pronounced inside the coaxial probe so steadier measurements can be obtained. On the negative side, coaxial probes are (1) mechanically difficult to fabricate with adequate precision, particularly concerning the centering and support of the open-air center conductor, (2) difficult to cut in custom lengths in the field, (3) difficult to ship in long sections, (4) difficult to join in short segments, (5) susceptible to blockage, and (6) difficult to make flexible for coiling during shipping.
A single wire transmission line, or Goubau line, overcomes most of the limitations to the coaxial probe and has been disclosed in U.S. Pat. No. 3,995,212, Apparatus and Method for Sensing a Liquid with a Single Wire Transmission Line, to Ross and U.S. Pat. No. 5,609,059, Electronic Multi-purpose Material Level Sensor, to McEwan. The key advantages to a single wire TDR probe for material level sensing are (1) extreme simplicity, (2) ability to coil the line for shipping (when made of wire), (3) simple custom cutting to length in the field, (4) nearly complete freedom from clogging (material can cling to the line, but generally has little effect), and (5) low cost.
A single wire probe requires a means to launch a TDR pulse onto the wire. A horn launcher, as described by Ross, exhibits high launching efficiency and provides a smooth impedance transition between the TDR unit and the high impedance of the single wire transmission line. However, the horn has notable disadvantages: (1) there is an impedance discontinuity that extends along the length of the horn that casts a distributed reflection and creates a potential measurement error, (2) there is no definite reflection to provide a xe2x80x9ctop-of-tankxe2x80x9d reference marker, (3) the horn ends too abruptly at its rim which creates a spurious reflection in the measurement range, (4) the horn is physically large and expensive, and (5) a large opening is needed to insert the horn through, often requiring a large, and therefore expensive, ANSI-rated tank cover.
(ANSI is the American National Standards Institute.)
A flat plate-type launcher, as described by McEwan in U.S. Pat. No. 5,609,059, creates a strong reflection to indicate the top of the tank, is mechanically simple, and does not require a large tank opening. Its primary disadvantages are (1) the launch point reflection is often too strong, creating pulse aberrations that extend into the measurement range, (2) it has a low launch efficiency relative to the horn, which results in excessively low signal returns from low dielectric constant materials, (3) due to its low launch efficiency, a hot ground condition exists that can propagate pulses backwards onto the outside of the TDR feed cable, creating spurious reflections and ringing.
A launcher is needed that combines the best performance features of both the horn and the plate with none of the drawbacks: good coupling efficiency, a controlled-amplitude marker reflection, absence of hot grounds, insertable through a small opening, and low cost.
Regardless of whether a coaxial or single wire line is used, it is most desirable to process the reflected pulses with automatic pulse detection techniques that render the measurement independent of pulse amplitude. McEwan, in U.S. Pat No. 5,610,611, High Accuracy Material Level Sensor, discloses a constant fraction discriminator, or CFD, that incorporates a peak detector to automatically set the trigger point on its pulse detectors. While this method eliminates pulse amplitude dependence, it suffers from dynamic errors that can arise in sloshing tanks. The dynamic errors arise from the inability of the peak detector to track rapid decreases in repetitive pulse amplitude. A new automatic pulse detector is needed, and preferably one which also rejects errors caused by low-frequency aberrations in the return signal.
Generally, the accuracy of commercial TDR-based material level sensors is on the order of 1%. In order to improve accuracy, the TDR timing system would need a stability on the order of a few picoseconds over time and temperature. Thus, a very precise pulse detection and timing system is needed that is not available in the prior art.
The present invention is a time domain reflectometer (TDR) having a single wire transmission line which is inserted into a tank or container, wherein the round trip travel time of reflected pulses indicates the location or, equivalently, the fill-level of the tank. Accurate measurements are made by measuring the difference in reflection times between a reflection at the top of the tank (designated T herein) and a reflection from the material in the tank (designated M herein). This Txe2x88x92M time difference is independent of interconnect cable lengths and propagation delays in the TDR apparatus. Consequently, accurate, stable measurements are possible at the picosecond level. The present invention is also a number of individual components used in the TDR.
In order to launch a pulse onto a single wire transmission line, a pulse launcher is needed, such as a coaxial horn or a well-grounded metal plate as used in the prior art. The present invention advantageously employs a sparse, open horn formed of several wires or leaves in place of the prior art pulse launchers to (1) provide a sharp, controlled-impedance discontinuity and thus a sharp, controlled-amplitude reflection, (2) efficiently launch a pulse onto the line, and (3) provide a smooth transition from the horn to free space to avoid spurious reflections at the horn rim.
An efficient pulse launcher, as provided by the present invention, virtually eliminates a hot ground effect commonly seen with plate-type launchers. With the open-wire horn, TDR pulses are partially reflected back to the TDR apparatus and partially transmitted onto the dipstick, and very little propagates backwards over the outside of the wire horn launcher and onto the outer jacket of the feed coaxial wire. Were this to occur, ringing and spurious reflections can usually be observed in combination with the desired reflections, making accurate measurements impossible.
Mechanically, the wire horn is simple, robust, and inexpensive. Notably, its wires can be bent inwards, in a similar fashion to folding an umbrella, so it can be inserted through a small tank opening such as a xc2xdxe2x80x3 threaded pipe opening. This feature greatly expands the range of applications for the present invention, such as for monitoring the oil level in standard 200 gallon heating oil tanks used throughout the northern U.S., which are commonly fitted with several top-side pipe-threaded openings.
In the present invention, a squarewave pulse is transmitted by the TDR apparatus and the return reflections are differentiated into impulses and subsequently sampled to produce an equivalent time (ET) video signal that is an exact replica of the realtime pulses, except on a vastly expanded time scale. Equivalent time techniques convert nanosecond events to millisecond events for vastly simplified processing.
The present invention includes a novel low aberration TDR pulse generator having one sharp edge used for measurement, and one slow, return-to-zero edge that has no effect on the system. In addition, a novel TDR circuit is employed to convert the transmitted TDR squarewaves to sharp impulses for accurate, time-of-peak measurement. As a further feature, a novel 2-diode sampler with extremely low line loading and blowby is utilized.
Amplitude-gated time-of-peak (TOP) detectors are employed to accurately detect reflected pulses and trigger timing counters. The TOP detectors are independent of pulse amplitude, and are accordingly independent of material dielectric constants, pulse risetime, pulse amplitude, manufacturing variations, long-term drift, and low frequency ringing.
In one embodiment, the accuracy of the system is further improved with a unique two-frequency, crystal-controlled timing system that yields scale-factor stabilities limited by the accuracy of a quartz crystal, which is typically xcx9c0.001%. Alternatively, the quartz crystal may be replaced with a temperature compensated crystal oscillator (TCXO), an ovenized crystal oscillator, or an atomic clock, all of which can provide stabilities well below 1 ppm/xc2x0 C.
The present invention can be used as an electronic dipstick for innumerable applications in material level sensing in containers. In combination with a valve, it can be used to control or automatically regulate the level in a toilet tank, for example. In a totally different application, it can sense the presence and location of an object in contact (or near contact) with its Goubau line, such as a security wire around a window. As a linear displacement transducer, where a moveable reflecting object slides along the Goubau line, vehicle height can be sensed or hydraulic cylinder displacement can be measured for safety or automatic control.